A variety of techniques have been developed and used for stimulating the production of oil and gas from subterranean formations penetrated by wellbores. One commonly used technique for stimulating producing formations formed of relatively low permeability materials comprises pumping a fracturing fluid at a pressure and rate into the formation whereby one or more fractures are hydraulically created. The fractures are extended by continued pumping, and a propping agent such as sand transported by the fracturing fluid is deposited in the fractures. The propping agent functions to maintain the fractures open after the hydraulic pressure on the formation is withdrawn.
Another commonly used production stimulation technique is known in the art a fracture acidizing. Fracture acidizing consists of creating and extending one or more fractures followed by etching the fracture faces with acid so that when hydraulic pressure on the formation is withdrawn, flow channels remain through which desired fluids contained in the formation flow to the wellbore.
While most wells are completed in a zone of best possible oil and/or gas production, it has heretofore been difficult to prevent a created fracture or fractures from extending vertically above and/or below the desired zone, often resulting in the fractures extending into less desirable zones in the formation or into adjacent formations. For example, zones capable of producing excessive water often lie adjacent to or are in close proximity to preferred production zones. When fracture treatments are carried out to stimulate the production of oil and/or gas from the preferred zones, the created fractures can extend into water producing zones resulting in the production of undesirable water along with desired oil and/or gas.
A number of techniques have been proposed for controlling the growth of fractures. These have met with varying degrees of success. For example, U.S. Pat. No. 3,335,797 issued Aug. 15, 1967 discloses a method of controlling the direction of fractures created during hydraulic fracturing where a propping agent is caused to be placed at the bottom of the fractures to inhibit subsequent downward fracturing during the extension of the fractures. U.S. Pat. No. 3,954,142 issued May 4, 1976 is directed to methods of confining a subterranean formation treatment such as an acidizing treatment to a desired zone within the formation by controlling the density of the various fluids involved. U.S. Pat. No. 4,509,598 issued Apr. 9, 1985 is directed to a method of limiting the upward growth of vertical fractures during a hydraulic fracturing treatment by including buoyant inorganic diverting agent in the fracturing fluid.
U.S. Pat. No. 4,515,214 issued May 7, 1985 is directed to a method for controlling the vertical growth of hydraulic fractures wherein the fracture gradients of the formation to be fractured and adjacent formations are first determined. Based on the fracture gradients, the density of fracturing fluid necessary to inhibit fracture propagation from the formation to be fractured into adjacent formations is determined. Afterwards, a fracturing fluid of such density is used to fracture the formation. U.S. Pat. No. 4,478,282 issued Oct. 23, 1984 is directed to a technique for controlling vertical height growth of fractures wherein a flow blocking material is utilized which forms a barrier to fluid flow into the vertical extremities of the fractures.
Lord et al. in U.S. Pat. No. 4,887,670 issued Dec. 19, 1989 teaches a method for controlling the growth of one or more vertically oriented fractures in a subterranean formation during a fracturing treatment. A first fluid having a known density and containing a fluid which may contain a proppant and having a known density different from the first fluid is next introduced into the fracture whereby the second fluid selectively overrides or underrides the first fluid. This fluid control causes the proppant to screen out and impede further downward or upward growth of the fracture.
What is needed is a method which limits vertical fracture propagation downwardly while further minimizing the fracture's intrusion into an underlying zone.